Perforating guns are used to complete an oil or gas well by creating a series of tunnels through the casing into the formation, allowing hydrocarbons to flow into the wellbore. Such operations can involve multiple guns that create separate perforations in multiple producing zones where, each gun is fired separately. Operations can also involve single or multiple guns in conjunction with setting a plug. The guns are typically conveyed to the producing zone(s) by wireline, tubing or downhole tractors.
Switches are typically coupled to each detonator or igniter in a string of guns to determine the sequence of firing. One type of switch uses a diode that allows two guns (or a gun and a plug) to be fired, one with positive and the other with negative voltage. Percussion switches are mechanical devices that use the force of detonation of one gun to connect electrically to the next gun, starting with the bottom gun and working up, and are typically used to selectively fire three or more guns. The devices also disconnect from the gun just fired, preventing the wireline from shorting out electrically. A problem with percussion switches is that if any switch in the string fails to actuate, the firing sequence cannot continue, and the string must be pulled from the wellbore, redressed and run again.
More recently, electronic switches have been used in select-fire guns. Unlike percussion-actuated mechanical switches, selective firing of guns, continues in the event of a misfired gun or a gun that cannot be fired because it is flooded with wellbore fluid. One commercial switch of this type has downlink communication but is limited in the number of individual guns that can be fired in one run. As with the percussion switches, the system relies on detecting changes in current at the surface to identify gun position, which may not be a reliable method to identify gun position in a changing environment.
Another type of electronic switch has both downlink and uplink communication and is not as limited in total number of guns that can be filed in a, run, but is somewhat slow to fire because of the long bi-directional bit sequence required for communication. Both downlink and uplink communications use a unique address associated with each switch to identify correct gun position prior to firing.
A common problem in operating downhole devices is keeping unwanted power from causing catastrophic action. Examples include a perforating gun receiving voltage that accidentally fires the gun downhole, a setting tool being activated prematurely, a release device suddenly deploying, and high voltage destroying electronics in a well logging tool because the power rating is exceeded. A solution to this problem is to stop unwanted power by inserting a blocking mechanism between the power supply and the downhole device to be protected. In a standard perforating job, the power to log and to detonate the perforating gun is located at the surface. Power can also be generated downhole using batteries. Recently, there have been detonator designs that incorporate electronics to block unwanted power from firing a gun.
The high voltage needed to power a downhole tractor presents particular problems protecting the tool string conveyed by the tractor. The surface voltages powering a tractor are typically 1500 VDC or 1000 VAC. Tractors normally have an internal design that prevents tractor power from being transmitted below the tractor, but sometimes the circuitry fails or does not work properly, allowing induced voltage or direct voltage to pass through the tractor into the tool string below. To protect the tool string, which can include perforating guns or logging tools, one or more special safety subs are located between it and the tractor. Some of the subs use electrical/mechanical relays to block accidental tractor power; others use electronic switches that are commanded to turn off and on using communication messages from the surface that contain a unique address.
More recently, the American Petroleum Institute (API) has issued a recommended practice for safe tractor operations, RP 67, that includes a recommendation that the tractor be designed to block unwanted voltage from passing through and that the design is free of any single point failure. In addition, there must be an independent, certified blocking device between the tractor and any perforating gun to prevent unwanted power from being applied to a gun.